V2 Make FileId type opaque and use consistently throughout project (#4307)

# Description of Changes
The `FileId` type in V2 currently is just defined to be a string. This
makes it really easy to accidentally pass strings into things accepting
file IDs (such as file names). This PR makes the `FileId` type [an
opaque
type](https://www.geeksforgeeks.org/typescript/opaque-types-in-typescript/),
so it is compatible with things accepting strings (arguably not ideal
for this...) but strings are not compatible with it without explicit
conversion.

The PR also includes changes to use `FileId` consistently throughout the
project (everywhere I could find uses of `fileId: string`), so that we
have the maximum benefit from the type safety.

> [!note]
> I've marked quite a few things as `FIX ME` where we're passing names
in as IDs. If that is intended behaviour, I'm happy to remove the fix me
and insert a cast instead, but they probably need comments explaining
why we're using a file name as an ID.

frontend/src/components/viewer/Viewer.tsx

@@ -15,6 +15,7 @@ import { fileStorage } from "../../services/fileStorage";
 import SkeletonLoader from '../shared/SkeletonLoader';
 import { useFileState, useFileActions, useCurrentFile } from "../../contexts/FileContext";
 import { useFileWithUrl } from "../../hooks/useFileWithUrl";
+import { FileId } from "../../types/file";
 
 
 // Lazy loading page image component
@@ -152,7 +153,7 @@ const Viewer = ({
   const { selectors } = useFileState();
   const { actions } = useFileActions();
   const currentFile = useCurrentFile();
-  
+
   const getCurrentFile = () => currentFile.file;
   const getCurrentProcessedFile = () => currentFile.record?.processedFile || undefined;
   const clearAllFiles = actions.clearAllFiles;
@@ -378,7 +379,7 @@ const Viewer = ({
         }
         // Handle special IndexedDB URLs for large files
         else if (effectiveFile.url?.startsWith('indexeddb:')) {
-          const fileId = effectiveFile.url.replace('indexeddb:', '');
+          const fileId = effectiveFile.url.replace('indexeddb:', '') as FileId /* FIX ME: Not sure this is right - at least probably not the right place for this logic */;
 
           // Get data directly from IndexedDB
           const arrayBuffer = await fileStorage.getFileData(fileId);